Like most things, and especially things relating to relationships, it depends. You might expect that it would offer a chance to find out "how compatible" you are, but that's a dangerous game to play - that's what the Honeymoon Period is designed to address: the shock of finding out just how different the real person you married is, from the romanticised ideal you thought you were dating. By the time reality gradually sets in, you've had a chance to come to terms with it during the "grab-and-go" initial period of living together, and then getting a divorce is such a pain, it's often easier to find ways to deal with it. Living together before marriage bypasses the honeymoon phase for many of us, since we're still living our regular lives, and then there's no divorce to act as an incentive to hold on and see if it can be worked out - so it's only too easy to leave in a snit, a tiff or even high dudgeon over things that are relatively minor. On the other hand, the other effect of the honeymoon period (that being pregnancy) can also come into play. Still, it's not necessarily a bad idea for many of us; some couples find out their tolerance limits only when living together, and it's probably better to do that before the marriage than afterwards. It's also pretty useful for exposing some of the more critical "incompatibilities", especially with regard to sex. That's one of the most powerful forces in a relationship - ranking right up there with money; if it's good and stays good, almost anything else can be tolerated. If it's not good, everything else better be pretty amazing.
Still, overall, I'd go with "generally not a good idea", but with the understanding that every couple is a unique combination, and it's hard to learn if you don't make mistakes.